1. Field of the Invention
This invention relates to a lubricating oil composition having improved antiwear performance due to the presence of a mixed ligand metal complex and a metal thiophosphate.
2. Description of Related Art
Engine lubricating oils require the presence of additives to protect the engine from wear. For almost forty years, the principal antiwear additive for engine lubricating oils has been zinc dialkyldithiophosphate (ZDDP). However, ZDDP must be used in concentrations of 1.4 wt. % or greater to be effective. Since phosphates may result in the deactivation of emission control catalysts used in automotive exhaust systems, a reduction in the amount of phosphorus-containing additives (such as ZDDP) in the oil would be desirable. In addition, ZDDP alone does not provide the enhanced antiwear protection necessary in oils used to lubricate today's small, high performance engines.
Certain mixed ligand metal complexes are known. For example, U.S. Pat. No. 4,308,154 discloses the use of mixed metal salts of dialkylphosphorodithioic acids and carboxylic acids in lubricating oils. Mixed ligand metal complexes have also been described in the literature (see C. A. Tsipis et al., "Mixed Ligand Iron (III) Dithiocarbamates", Inorg. Chem. Acta., 23, p. 163 [1977]; R. W. Gable et al., "Preparation, Properties, and Structure of Di-.mu.-thio-bis[bis(O-ethyl dithiocarbonato] tin (IV)" J. C. S. Dalton, p. 1392 (1981), and B. F. Hoskins et al., "The Preparation and Characterization of Mixed Dithiolate Ligand Complexes", Inorg. Chem. Acta, 105, p. 171 (1985)). Adding physical mixtures of various ligands to lubricating oils are also known (see, for example, U.S. Pat. Nos. 4,171,268; 4,395,343; and 4,402,840).
However, none of these publications suggest that the antiwear performance of a lubricating oil can be enhanced when certain mixed ligand metal complexes and a metal thiophosphate are present therein.